DESCRIPTION (Investigator's Abstract): Three specific aims will be the focus of this continuing study of motor disability in stroke-induced spastic hemiparesis. The first specific aim will attempt to distinguish two possible mechanisms underlying the disturbance in spatial patterns of muscle selection in hemiparetic stroke. These two are the anatomical constraints imposed by branching patterns of residual ventromedial spinal pathways, or increased sensitivity of interneuronal pathways in the flexion-reflex. These two alternatives will be examined using cutaneous stimuli, cutaneous anesthesia, and single motor unit recording techniques. Limited or absent modification of EMG spatial pattern in response to manipulation of cutaneous input would argue for a pre-eminent supraspinal role. The second aim will be to characterize the disturbances in voluntary movement in the spastic paretic limbs by evaluating the spatial characteristics of movement to different locations in the horizontal plane, while recording movement velocity, acceleration and limb damping characteristics. The results of these studies should allow to voluntary motion and whether damping of voluntary motion is in part responsible for the disability of hemiparetic stroke. The third aim is to determine whether muscular weakness in stroke is mediated, at least in part, by disturbances of motor unit recruitment and rate modulation within individual muscles. These abnormalities are readily apparent in animal models of spastic paresis, where they appear to reflect disturbances in neuromodulator input to the spinal motoneurons. Single motor unit recording techniques will be used to compare recruitment, rate characteristics, and synchrony of motor units in matched muscles on the paretic and contralateral sides of the same hemiparetic subject. Taken together, these studies should advance our knowledge of the mechanisms underlying the disturbances of movement in hemiparetic stroke and should help provide guidelines for rational diagnosis and therapeutic intervention.